1. Field of the Invention
The present invention relates to optical systems and is suitable for optical systems included in optical apparatuses such as silver-halide-film cameras, digital still cameras, video cameras, digital video cameras, telescopes, binoculars, projectors, and photocopiers.
2. Description of the Related Art
Optical systems included in optical apparatuses such as digital cameras, video cameras, and projectors are desired to have a wide angle of view and form a high-quality image without distortion.
Wide-angle lens systems are known as optical systems suitable for obtaining an object-side image of a wide range in a natural state without distortion. In a wide-angle lens system known as a so-called normal-projection, wide-angle lens system, when the image height, the focal length of the system, and the angle of view (an angle at which an object is viewed along an optical axis) are denoted as Y′, f, and ω, respectively, the following relationship is satisfied: Y′=f·tan ω.
Usually, to effectively realize a wide angle of view, a wide-angle lens system includes a negative lens element or a negative lens unit (in Background of the Invention, lens element and lens units are collectively referred to as “lenses”) having a strong negative power (refractive power) near a magnification-conjugate side. To widen the angle of view in a lens system having such a configuration, the negative power of the negative lens needs to be increased.
In general, as the power of the negative lens is increased, the degree of negative distortion also increases. Therefore, it becomes difficult to satisfy the relationship of Y′=f·tan ω.
In contrast, there are normal-projection, super-wide-angle lens systems, such as those disclosed in Japanese Patent Laid-Open Nos. 2001-159732 (corresponding U.S. Pat. No. 6,621,645) and 2001-188171, having an imaging angle of view of 2ω>100°. In these lens systems, distortion is effectively corrected by providing an aspherical lens near a negative lens having a strong power.
In the super-wide-angle lens system disclosed in Japanese Patent Laid-Open No. 2001-159732, the negative lens having a strong negative refractive power and provided near the magnification-conjugate side is composed of a material having a high refractive index and a low dispersion. Thus, a wide angle of view is effectively realized while lateral chromatic aberration occurring in a positive lens provided near a demagnification-conjugate side is effectively corrected.
In general, a wide-angle lens system has a configuration in which lenses are arranged asymmetrically, in terms of refractive power, with respect to an aperture stop.
A wide-angle lens system having such an asymmetrical refractive-power arrangement tends to cause large lateral chromatic aberration as well as negative distortion (barrel-shaped distortion).
There are other wide-angle zoom lens systems such as those disclosed in Japanese Patent Laid-Open Nos. 2001- and 2006-084971 (corresponding U.S. Pat. No. 7,079,324). In these zoom lens systems, such lateral chromatic aberration is corrected by using appropriate optical materials.
In the wide-angle zoom lens system disclosed in Japanese Patent Laid-Open No. 2001-166206, a positive lens provided near the demagnification-conjugate side is composed of glass having anomalous partial dispersion so that occurrence of lateral chromatic aberration for short wavelengths is suppressed.
In the zoom lens system disclosed in Japanese Patent Laid-Open No. 2006-084971, a negative lens provided nearest the magnification-conjugate side is composed of a highly refractive material having a large second-order dispersion. This offsets insufficient correction of lateral chromatic aberration for short wavelengths.
Most of currently available highly refractive optical materials having a large second-order dispersion are high-dispersion materials.
If such a material is used, lateral chromatic aberration for wavelengths near the dominant wavelength (the d-line) tends to be overcorrected. Such a tendency is utilized in the zoom lens system disclosed in Japanese Patent Laid-Open No. 2006-084971, in which the positive lens provided nearest the magnification-conjugate side is composed of a high-dispersion material, whereby lateral chromatic aberration for wavelengths near the dominant wavelength is corrected well.